Polymeric materials are generally considered useful as viscosification agents when dissolved in an appropriate solvent system. The major reason for this viscosity enhancement is due to the very large dimensions of the individual polymer chain as compared to the dimension of the single solvent molecules. Any increase in size of the polymer chain will produce a corresponding enhancement in the viscosity of the solution. This effect is maximized when the polymer is dissolved in a "good" solvent. Therefore, in general, a hydrocarbon soluble polymer is useful for thickening hydrocarbon solvents, while a water soluble polymer is appropriate for increasing the viscosity of aqueous systems. With regard to aqueous systems, polyelectrolytes (e.g., hydrolyzed polyacrylamide) are very useful and commonly used materials; however, the solution properties of these materials begin to deteriorate as ionizable additives (i.e., acids, bases or salts) are dissolved in the solution. These additives screen the charges that are fixed along the polymer backbone which results in a decrease in the dimensions of the polymer molecule. The viscosity diminishes as long as the chain continues to shrink. Additionally, polyacrylamide-type polymers suffer from poor hydrolytic or thermal hydrolytic stability. This hydrolytic instability becomes especially undesirable in the presence of divalent cations because polymer precipitation occurs. Moreover, polyacrylamide polymers build viscosity by increasing polymer molecular weight. Increasing molecular weight leads to increasing mechanical shear instability.
N-vinyl pyrrolidone (NVP) homopolymers are well known and reviewed by Davidson and Sittig in Water-Soluble Resins. Such homopolymers are characterized by good hydrolytic stability, even in the presence of electrolytes, but poor viscosification efficiency (viscosity per unit concentration) and undesirable adsorption on rock. (H. P. Frank, J. Polymer Sci., 12, 5;65 (1954); A. Conix, J. Polymer Sci., 15, 221 (1955); G. A. Stahl, European Patent Application No. 84100918.6 (Jan. 30, 1984).
Copolymers of NVP with acrylamide (AM) have also been disclosed. (A. M. Chatterjee and C. M. Burns, Canadian Journal of Chem., 49, 3249 (1971); G. A. Stahl, European Patent Application No. 84100918.6 (Jan. 30, 1984). Such materials have improved hydrolytic stabilities. However, the viscosification efficiency of such materials is still low.
Acrylamide copolymers containing small amounts of long chain C.sub.6 -C.sub.20 N-alkylacrylamide show enhanced viscosification efficiency, while suffering the debit of relatively poor hydrolytic stability (e.g., Bock, et al., U.S. Pat. No. 4,520,182). Bock further noted that special microemulsion or micellar polymerization techniques were required to produce soluble copolymers of acrylamide and long chain N-alkylacrylamide (e.g., n-octylacrylamide) for use as efficient viscosifiers. (See also U.S. Pat. No. 4,528,348 and U.S. Pat. No. 4,521,580, both filed on Aug. 29, 1983).
G. A. Stahl (European Patent Application No. 84100918.6, Jan. 30, 1984) broadly teaches terpolymers of NVP, AM and minor amounts of a third monomer selected from the group of hydrophobic compounds as vinyl pyridines, hydroxylated esters of ethylenically-unsaturated carboxylic acids and N- and N,N-alkylacrylamide, where the alkyl group contains more than 2 carbon atoms. (Stahl, page 23, line 15 to page 24, line 25). While these monomers are hydrophobic, some are water dispersible or even water soluble (e.g., isopropyl acrylamide) and some are water insoluble (e.g., n-octylacrylamide). Thus, the critical distinction between water soluble (dispersible) hydrophobic monomers and water insoluble hydrophobic monomers was not made. Moreover, no terpolymers containing the highly hydrophobic (insoluble) long chain N-alkylacrylamide were exemplified, possibly because of the difficulty in incorporating such monomers into homogeneous water soluble terpolymers.
For example, Bock in U.S. Pat. No. 4,520,182 notes that the:
synthesis of copolymers composed of water soluble and water insoluble monomers [e.g., long chain N-alkyl acrylamides] presents difficulties.
In order for polymerization to be effected, the monomers must obviously come into close proximity to one another . . . .
For example, simply dispersing the water insoluble monomer as fine particles in the aqueous medium while dissolving the water soluble monomer in water would result in poor incorporation of the water insoluble monomer and would lead to a heterogeneous product polymer (Bock, U.S. Pat. No. 4,520,182, column 1, lines 40-57).
The instant tetrapolymers show enhanced viscosification, hydrolytic stability and/or mechanical stability compared with other typical polymers of the art.